Since the discovery of carbon nanotubes (CNT) in 1991, much has been done to characterize their properties and explore their potential applications. Currently, carbon nano particles including both nanotubes and monofilaments are found in extended commercial applications in modern technologies, for example, for manufacture of composite materials, nanoscale machines, flat panel displays, and computer memory devices. The wide application of carbon nanotubes is based on their unique physical and mechanical properties, which show the high electrical and thermal conductivity, and high strength values along the nanotubes' axis.
Their high aspect ratio, mechanical resilience and electrically conducting make them ideal for probe microscopy tips. There are several different types of scanning probe microscopy, including scanning tunneling microscopy (STM), scanning force microscopy (SFM), atomic force microscopy (AFM), magnetic force microscopy (MFM), and magnetic resonance force microscopy (MRFM). Nanotubes have previously been made into atomic force microscopy (AFM) tips and have proven to have great advantages in imaging and manipulation over conventional silicon and silicon nitride tips. AFM instruments are well known for producing images with resolution in the nanometer or smaller range. AFM resolution is dependent on physical characteristics of the scanning probe including composition, size, shape and rigidity of the probe. Both length and width (or diameter) of the probe affect the resolution because, for example, the length limits the maximum depth of a detail that may be measured, and the width limits the minimum breadth of a detail that may be measured. Silicon probes are commonly used, but have a tip diameter generally greater than 10 nm, and are easily damaged or worn during use. Scanning probes made of carbon nanotubes have been shown to be acceptable alternatives to silicon probes and are known to be mechanically stable.
However, there are no easy and controllable methods to attach a carbon nanotube to a scanning probe tip, due to the extremely small size of the carbon nanotubes. Previous approaches have included the mechanical attachment of a CNT onto an AFM tip, chemical vapor deposition growth of a CNT directly onto commercial atomic force microscope made of Si or one of its derivatives, and electric or magnetic field induced multiwall nanotube probe attachment.
Mechanical attachment of nanotubes on a scanning probe tip using optical microscope was developed in 1996. See Dai, Hafner, Rinzler, Golbert and Smalley, Nature 384, 147 (1996). In this process, micromanipulators are used to control the positions of a commercial cantilever tip and a bundle of nanotubes, while viewing with an optical microscope. This approach has allowed the initial development of nanotube tips although it has significant limitations. Firstly, the assembly procedure inherently selects towards thick bundles of nanotubes since these are easiest to observe in the optical microscope Bundles are selected because it is extremely difficult to observe an individual nanotube due to its nanometer size. However, mechanical assembly of nanotube tips has also been performed inside a scanning electron microscope (SEM). The use of the SEM still limits assembly to nanotube bundles or nanotubes with diameters greater than 5-10 nm and, moreover, increases greatly the overall time required to make one tip. Secondly, well defined and reproducible tip etching procedures to expose individual nanotubes do not exist. Thirdly, a relatively long time is required to attach nanotubes to commercial cantilevers, thus increasing the cost.
To overcome the limitations of mechanical attachment of nanotubes, direct growth of carbon nanotubes on the tip by chemical vapor deposition (CVD) was developed in 1999. See Hafner, Cheung, and Lieber, J. Am. Chem. Soc. 121, 9750 (1999). In this technique, a flattened area was created at the cantilever tip, pores were etched into the flat area and a catalyst was deposited into the pores. CVD was then used to allow nanotubes to grow from the tip in alignment with the pores. This method produced thin, individual multiwall CNT tips, however there was no control over the orientation of the nanotubes. For SPM tip application, the nanotube has to be perpendicular to the surface.
Alternative methods to attach a carbon nanotube to a scanning probe tip were electric field induced attachment which was developed in 2000 and magnetic field induced alignment which was developed as in 2003. See Stevens, Nguyen, Cassell, Delzeit, Meyyappan, and Han, Appl. Phys. Lett., 77 3453 (2000) and Hall, Mathews, Superfine, Falvo, and Washburn, Appl. Phys. Lett., 82, 2506 (2003). In these technique, electric or magnetic fields are used to align the carbon nanotube along the scanning probe tip axis. The electric or magnetic field induced attachment provides advantages in adjustment of the orientation of the nanotubes, however, the methods require precoating to improve conductivity. The four above described methods were somewhat arduous have met with difficulties in adhesion, reproducibility, and process control. It has been demonstrated already that carbon nanotubes (CNT) are excellent scanning probe tips. However, as discussed above, there are no easy and controllable methods to attach a carbon nanotube to a scanning probe tip, due to the extremely small size of the carbon nanotubes.
The present invention contributes a more consistent and controlled method for attaching a novel carbon nanotube probe to the SPM cantilever tip using Focus Ion Beam (FIB) technology. Through the method of the present invention, a FIB tool is used to form a slot in the SPM cantilever tip and the carbon nanotube probe is inserted into the formed slot. The inserted carbon nanotube probe is welded to the SPM cantilever tip using the FIB tool to deposit metal atoms to the joint between the carbon nanotube probe and the cantilever tip, thus welding the carbon nanotube probe to the cantilever tip.